The packaging of integrated circuits for mm-wave communication, radar and imaging systems has long been dominated by the use of expensive specialised technologies like low loss ceramics and thin-film processes. In addition, such packages tend to be mounted on specialised mm-wave substrates on which the antenna is integrated. An important motivation for the dedicated packaging technology and substrates is the need for a well defined interconnection between the transceiver and the antenna that offers low loss and low parasitic effects at mm-wave frequencies. Other requirements are a low thermal resistance between the active devices and the application board and mechanical reliability. The main motivation for the use of specialised low loss and low dielectric constant materials for antenna substrates relates to the good efficiency and impedance bandwidth characteristics achievable with such materials.
For the purposes of the present disclosure, the term millimeter-wave refers to radiofrequency signals covering a range of wavelengths generally of the order of millimeters, i.e. with a typical frequency range of 30 to 100 GHz (corresponding to 10 to 3 mm in wavelength). This particular range of wavelengths defines the dimensions of the antennae that are used, and therefore the particular packaging that is possible for transceiver systems operating over the relevant range of frequencies.
Millimeter-wave applications are at present entering mass consumer markets, resulting in a strong drive for low cost alternatives for packaging and board solutions. A major cost reduction can be achieved by integrating an antenna with the package, which relaxes the interconnect requirements on the board significantly since the mm-wave interconnect is then limited to the connections between the transceiver and antenna in the package, rather than through wired connections between different packages. Known packaging concepts do however still tend to require special expensive packages in order to integrate an antenna function.
One of the most popular antenna types for use in wireless communication, radar and imaging equipment is the micro-strip antenna. This type of antenna is a low profile, mechanically simple, robust and inexpensive structure that can be used as a single element radiator, but can also be used to construct linear and planar conformal antenna arrays [1]. Important specifications for such applications include: i) the impedance bandwidth, which defines the efficiency of the transfer of available source power to the antenna; and ii) the gain, which defines the efficiency of the conversion of this power to radiated power and the directivity of the radiated waves.
The impedance bandwidth of a resonant antenna is a function of the effective antenna volume. This volume is strongly dependent on the thickness and permittivity of the antenna dielectric. The following equations show the general relationship between impedance bandwidth and effective antenna volume [2]:
                              Q          ant                >                  η          ·                      (                                          1                                                      (                                          k                      ,                      r                                        )                                    3                                            +                              1                                  (                                      k                    ,                    r                                    )                                                      )                          ≈                  η          ·                      (                          1                                                (                                      k                    ,                    r                                    )                                3                                      )                                              (        1        )                                BW        =                              vswr            -            1                                              Q              ant                        ·                          vswr                                                          (        2        )            
where Qant is the minimum antenna quality factor, η is the antenna efficiency, k is the wave number (k=2π/λ) and r is the radius of the smallest sphere circumscribing the antenna.
FIG. 1 (derived from reference [3]) shows the impedance bandwidth of a standard square shaped patch antenna as function of substrate thickness and dielectric constant. As the substrate thickness increases, the 2:1 VSWR Bandwidth increases. The quantity on the x-axis of FIG. 1 (and also on FIG. 2) is the ratio of the substrate thickness t to the free space wavelength λ, resulting in the dimensionless quantity of relative substrate thickness, t/λ. The different curves shown in FIG. 1 represent results for different values of relative dielectric constant, ranging from 1.0 (for air/free space) to 9.8.
The gain, G, of an antenna is defined by the product of efficiency and directivity, given by the following equation:G=η·D  (3)
in which η is the power efficiency and D the directivity of the antenna. The directivity is mainly defined by the spatial distribution and orientation of the radiators, ground-planes and substrates (or superstrates). The efficiency of micro-strip antennas is not only defined by dielectric and conductive material losses but also by the power lost in the surface wave that can be launched in the substrate. The energy in this wave, which is partly transmitted into free space at the substrate edges and partly dissipated in the substrate, is generally considered as lost energy since it does not contribute to the radiation emitted in the desired direction (typically orthogonal to the plane of the antenna). FIG. 2 (also derived from reference [3]) shows the surface wave efficiency of a standard square shaped patch antenna, also as a function of the substrate thickness and dielectric constant. Again, an increase in dielectric constant results in increased losses due to surface wave losses.
The above mentioned results show a clear trade-off between impedance bandwidth, which increases with substrate thickness, and surface wave efficiency, which decreases with substrate thickness. The results also show that both bandwidth and efficiency can be improved by reducing the dielectric constant of the substrate.
Various technology and assembly methods are known at present to realise micro-strip antennas that are optimized with respect to impedance bandwidth and efficiency, for example as disclosed in reference [4], in which a laminate having a low dielectric constant and low loss at microwave frequencies is used as a dielectric between a patch antenna and a ground-plane. This type of structure is illustrated in FIG. 3, in which a patch antenna 301 is separated from a ground plane 302 by a dielectric medium 303, in this case composed of a PTFE-based composite material (known as ‘RT-Duroid’). The whole antenna package 300 is connected to an underlying substrate or motherboard 304 by means of solder balls 305 in the form of a ball grid array (BGA). Microwave ICs 306, 307 are mounted to the underside of the dielectric medium 303. This approach is typical for such antennas, where dielectric materials such as PTFE (polytetrafluoroethylene) are commonly used.
FIG. 4 shows a further antenna and transceiver module 400, proposed in reference [5], in which an air gap is provided between the antenna 401 and the ground plane 402. The antenna 401 is patterned on a fused silica dielectric superstrate 403. The superstrate 403 is bonded on to a metal frame 404, with the antenna 401 on the underside of the superstrate and electrically connected to a laterally offset transceiver IC 405 using a conductive adhesive. The antenna 401 is thereby suspended in air beneath the superstrate 403, and the metallic base plate 402 of the package 400 acts as a reflecting ground plane. The package 400 is connected to an underlying substrate (not shown) via a land grid array.
FIG. 5 shows a further antenna construction, shown in an exploded perspective view, as disclosed in reference [6]. In this case the bandwidth of the antenna is improved by the use of multiple parasitic resonators, which are coupled to the driven antenna patch 501 across multiple dielectric layers ∈r1 to ∈r8. The use of multiple resonators increases the order (number of poles) of the system and allows for a trade-off to be made between impedance bandwidth and the quality of the match. This structure is an example of a class of antennae employing multiple resonators either in the form of adding extra antenna elements or by combining single antenna radiators with lumped circuit resonators.
These known methods for realising a System in a Package (SiP) with an integrated micro-strip antenna have a number of disadvantages.
A general disadvantage of the known concepts is that the transceiver and antenna use different substrates as a result of the different requirements for the antenna and the transceiver. The substrate requirements for the transceiver typically include a high density flip-chip interconnect and a low thermal resistance. Requirements for the antenna, on the other hand, relate to the dielectric thickness, relative dielectric constant and material losses. The disadvantage of using different substrates in a SiP is that the resulting interconnect-structure is generally quite complex, resulting in efficiency and bandwidth losses due to parasitic effects.
In addition, a drawback of using special substrate (or superstrate) materials having a low dielectric constant and low loss is that such materials are in general incompatible with mainstream large volume mass production processes. PTFE-based materials such as RT-Duroid™ (∈r=2.9, tan δ=0.0012), Ultralam2000™ (∈r=2.5, tan δ=0.0022) or extruded polystyrene foam (∈r≈1.1, tan δ<0.002) are substantially more expensive due to the low production volumes and the requirement to accurately control and guarantee the dielectric properties up to high frequencies.
In addition, the mechanical and thermal properties of typical dielectric materials deviate substantially from other standard laminate materials such as epoxy, which makes the materials less suitable for standard assembly methods such as fine pitch flip-chip and ball grid array packaging.
Furthermore, the performance of current approaches is still limited by the dielectric constant of the material used, which is typically significantly higher that that of free space.